There is little doubt about the identity of the most vulnerable senator seeking re-election next year. It’s Illinois Republican Mark S. Kirk, who hopes to win a second term in a very Democratic state in a presidential year. His prospects are bleak.

But who ranks just behind Kirk as the second most vulnerable senator up next year?

Almost a month ago, Roll Call released its vulnerability list . Kirk was followed by Ron Johnson of Wisconsin, Patrick J. Toomey of Pennsylvania and Kelly Ayotte of New Hampshire. All three represent states that are politically competitive but were carried twice by Barack Obama.

Instead of simply accepting the Roll Call rankings, I decided to look for myself.

According to the Federal Election Commission, Obama’s 2012 margin in Pennsylvania was 5.4 points, compared to a margin of 5.6 points in the Granite State and a significantly larger 6.9-point margin in Wisconsin.

Four years earlier, in an even better Democratic year, Obama’s best margin in the three states was in Wisconsin (13.9 points), with Pennsylvania the next closest (10.3 points) and New Hampshire the narrowest (9.61 points). (See Appendix A of FEC report .)

The last Republican presidential nominee to carry New Hampshire was George W. Bush in 2000, though it was his narrowest state victory that year. Bush carried it by a little more than 1 point, with 48.1 percent of the total vote. (He lost the state four years later.)

Democrat Al Gore carried Wisconsin that year by fewer than 6,000 votes out of almost 2.6 million cast for a photo finish in the state. (Four years later, John Kerry carried it by just over 11,000 votes in another squeaker.) The last time Wisconsin gave its electoral votes to a Republican was in 1984.

Pennsylvania last went for a GOP nominee in 1988, when George H.W. Bush carried the state over Michael Dukakis by 2.3 points.

Clearly, all three states tilt Democratic at the presidential level, with Wisconsin’s bent the strongest.

But Republicans currently control both legislative chambers in each of the three states. Wisconsin has a two-term GOP governor, and Pennsylvania tends to flip back and forth between the two parties for the state’s top job. Democrats have controlled New Hampshire’s governorship since 2004.

Of the three Republican senators, Ayotte faces the most formidable opposition this cycle, at least if past electoral performance is any indication. Maggie Hassan won the state’s governorship by a comfortable 12-point margin in 2012 and was re-elected much more narrowly two years later.

Johnson will face the man he beat in 2010, former Sen. Russ Feingold, while Toomey will face either former Rep. Joe Sestak, whom he defeated in 2010, or Katie McGinty, who finished an uninspiring fourth in the 2014 Democratic gubernatorial primary. Subsequently, McGinty, a former Secretary of the state Department of Environmental Protection, became chief of staff to the Keystone state’s governor, Democrat Tom Wolf.

Ayotte annihilated her 2010 Democratic opponent, then-Rep. Paul W. Hodes, by 23 points in her Senate victory, while Johnson beat Feingold by just under 5 points and Toomey won an even narrower 2-point squeaker over Sestak.

Of course, Ayotte was running for an open seat. The Pennsylvania seat was also open, but only because Sestak defeated party-switching Sen. Arlen Specter in the Democratic primary. Johnson defeated Feingold, who had defeated an incumbent in 1992 before winning re-election in 1998 and 2004.

Johnson’s victory was more impressive than Toomey’s, considering the margins, the states’ past presidential performance and the two situations in 2010.

Clearly, a reasonable case can be made for any ranking of the three senators in terms of vulnerability in 2016.

My own inclination is to rank Johnson as the second most vulnerable incumbent, Toomey as third and Ayotte as fourth, exactly as Roll Call did. But that is now. I could have a very different view come Labor Day or, more importantly, at the end of October.

Johnson is the least approachable of the three, a self-assured (many would say “smug”) businessman who seems less interested in hearing what his constituents think and more concerned with shrinking government and cutting spending. Instead of attending to the needs of voters, he spent the first years of his term telling constituents that he didn’t go to Washington, D.C., to help them out but, instead, to save the country.

Toomey went out of his way to forge an alliance with West Virginia Sen. Joe Manchin III to introduce legislation that sought to find a middle ground on gun sales and background checks. That effort, which may or may not help Toomey appeal to voters in the southeastern corner of the state, demonstrates a political sensitivity that Johnson doesn’t have.

Ayotte’s style and gender are assets, making it more difficult for Democrats to demonize her as a tea party extremist or as “anti-women.” She is certainly conservative, but she clearly has differences with many in her party. Compromise is not a dirty word to her.

Unlike Johnson, she voted to ratify an international convention on the rights of people with disabilities in 2012 and a payroll tax/unemployment benefits extension compromise in late 2011.

Toomey voted with Johnson on the convention ratification issue but with Ayotte on the payroll tax/unemployment benefits extension. Ayotte, who signed on to support the Environmental Protection Agency’s Clean Power Plan, has been relatively moderate on environmental issues.

All three of the GOP senators could win — or lose. Together, they constitute the core of the fight for Senate control, which is why you should be watching those three contests particularly closely for the next 11 months.